Challenges of Scanning Hall Microscopy using Batch Fabricated Probes

Kodai Hatakeyama MSc
Presentation PhD defense
Date 2016-09-02
Time 14:30
Location University of Twente - prof.dr. G. Berkhoff room, building de Waaier 4

Scanning Hall probe microscopy is a widely used technique for quantitative high resolution imaging of magnetic stray fields. Up to now probes with nanometer spatial resolution have only been realized by electron beam lithography, which is a slow and expensive fabrication technique. In this thesis, we employ corner lithography to enable batch fabrication of high resolution scanning Hall probes.

The initial design consisted of a sub-μm Hall cross supported by four free standing wires in a pyramidal configuration, located at the end of an AFM type cantilever. This implementation was mechanically and electrically too fragile to operate. Therefor the design was improved by supporting the wires with a robust silicon-nitride membrane. These robust probes allowed for scanning operation, but we discovered that the output signals suffered from large topographic crosstalk. We determined that this crosstalk was caused by the combination of cross asymmetry, resulting from fabrication imperfections, and topography induced probe-sample distance modulation, leading to temperature variation.

To circumvent the crosstalk, we introduced an electronic compensation method to suppress the effect of temperature variation on the detected Hall signal. The method suppresses the temperature effect by at least a factor of ten, provided that the probe temperature varies uniformly over the entire structure. However, the method is not capable of  compensating temperature changes within the probe structure itself, for instance caused by asymmetric probe cooling at step edges on the sample, or cantilever torsion.

To increase the signal-to-noise ratio and improve the electrical robustness of the probes even more, we improved the probe design further by widening the leads to the Hall cross and increasing its dimensions. Using this final design, we successfully demonstrated imaging on a thermo-magnetically patterned magnetic sample with domains of 10 μm×10 μm, at a sensitivity of 4.12 V/T.

Posted on Monday, August 22, 2016